There are so many belts out there that it's impossible for a store to have everything in stock. If the belt in question is also applicable to something else, that might work, but if it's an oddball, probably not. They can order things if you can read the number in many cases.
I had a Saab that had a triple pulley in the back of the engine from which ran a single belt and a matched pair for various peripheral items -- water pump, ps pump, alternator, etc. I was able to get the matched pair after a week's wait through NAPA, but the single was a stock item. The closest Saab dealer to me is 35 miles or so and they wanted about triple the NAPA price for everything. How about $7 sparkplugs -- back in the early 90's??